4 Answers
4

The key to keeping your back happy is to drive as much pack weight to your hips as possible. A side note on weight is that the lighter your pack overall, the happier your back.

A lot of the packing order depends on your particular pack, but in general, the bottom of a pack is below your hips. Therefore, it is best to put something big and light in the bottom. A fleece is one idea, or your sleeping bag (if it's down, might be less great if synthetic due to the amount of crushing that will happen).

Now that we have the level of stuff above your hips, heavy things go next, right against your back. I like to put my food bag on one side and the tent on the other. Down the middle, cooking gear, maps, books, etc.

If there is room left away from your back, stuff your sleeping pad and random clothes.

On the top all this stuff, put whatever light items you have left (i.e. repair kits, first aid kit, rain jacket).

The key to this idea is that your heavy things are silo-like, i.e. tightly packed stuff sacks. That way their weight will be driven to your hips, and anything stacked on them will also drive weight to your hips. The "looser" your packing job, the less efficiently this will happen.

+1 I think the key is to have a predictable center of gravity. Heavy items that are too high or too low can have a tendency to throw you off balance. Keep the heavy items in the middle as close to your back as possible.
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Timothy StrimpleJan 24 '12 at 20:28

Well, this can somewhat depend on the type of backpack you have and the length of the trip you're planning to take (so how much you will be carrying), but there are a few general principles that apply to almost all situations:

From the bottom up:

The sleeping bag. Most backpacks have a larger, seperately-zipped area at the bottom that is the most convenient for storing a bulky item like a sleeping bag. You might also be able to fit other items that you will be using at the same time (for sleeping, at night) in that area.

The heaviest, bulkiest items. You need to pack the heaviest items on the lower/central side so that they will be closer to your back. If they are too high up they will pull the backpack away from your spine and put pressure on your shoulders. The closer to your spine the heavy items are, the easier it will be to balance your backpack and the weight will be positioned better over your hips. Usually the heaviest part of the backpack is your food, so that should go here.

Lighter items that don't need to be accessed often. Clothes, basically. You can also wedge clothes in around the heavy items (your stove and food) to prevent them from moving in your pack and keep the centre of gravity stable. Packing the tent body here is also good for keeping the heavy items in place.

Sleeping pads can be attached inside the lid of the pack or you can use some extra straps to strap it outside. If you have space, you can unwrap your sleeping pad and use it to line the inside of the pack, packing the rest of your items inside it. This helps to keep everything stable and also protects it from getting wet.

If you have space, you can seperate the tent body and the tent poles and pack them on the inside. You can stuff the tent body in at the botom of the pack and place the poles vertically across the length of the whole pack. Otherwise you might have to pack the tent in the lid.

Most packs have small outer compartments that are good for storing things you might need to access really easily whilst walking, like first aid kits and snacks.

One option that is super useful when you're travelling in a group is to have different packs for different purposes rather than each person carrying their own stuff. For example we have a "tent pack" and a "kitchen pack". The advantages include:

a significant weight difference, which you could use to support hikers of different strengths.

it's quicker to find something when you need it, because you know which pack it's in.

on arrival at camp, you put the packs on opposite sides of the site and there's less running around.

no concern about "food in the tent" if sleeping bags and food are not even in the same backpack.

we only need one super-waterproof large liner (for the tent pack) since it's ok if pots, dishes etc get wet (food is in smaller super-waterproofs.) Pretty much everything in the tent pack needs to stay dry.

If you're in a group of 2 or more, consider it. Also, if you don't live together and didn't pack together, the pause before setting off during which you reorganize everything into categories is a fantastic way to check that you've all brought the right stuff.

As mentioned in other answers, it will depend on the type of your backpack. You can for example pack like this:

First you unbuckle all straps of your backpack, and lay it on the ground. Next to it you spread your ground sheet or shelter half, fold your sleeping bag or blanket on top of it, so it will be about as wide as the central part of your backpack, and put your extra clothes, food for cooking in the evening and morning, and anything that you will not need during the day, like so:

Then you fold the top and bottom parts of the sheet over your sleeping bag or blanket,and roll everything up tightly. Place the roll on the central part of the bag (if the roll is too large, you have too much stuff. Get rid of some of it), like so:

Fold the bottom flap up over the roll, fold the side flaps over the roll as well, thread the side straps through the middle strip on the bottom flap, and buckle them to the other side strap. Then fold the top flap with a detachable compartment down, and buckle the long bottom strap to it.

Place items that you will need on the trail, but don't want to keep in your pockets (map, snacks, emergency kit, rain poncho, ...) into the top detachable compartment. Strap your pot, canteen, hatchet, knife etc to the outside of the pack with extra straps, and you are ready to go. A nice example of the finished product can be found here.